1. Field of the Invention
The invention relates generally to optical amplification, and more particularly to temperature control in optical amplification.
2. Description of the Prior Art
Optical fibers are an excellent communication medium, e.g. in avionics, because optical fibers provide large bandwidth, are lightweight, and are immune to electromagnetic interference. In some environments such as the aircraft environment, some challenges for optical amplification system design exist that are not currently addressed in the communications markets. These challenges include the need for low power consumption and a wide operating temperature range.
Standard erbium doped fiber amplifiers (EDFAs) are designed for operation between 0° C. and 70° C. The erbium doped fiber amplifiers operate over this range using either uncooled pump lasers or pump lasers operated at a specific temperature, (e.g. 25° C.) through use of a thermoelectric cooler.
When an optical amplification system operates at a temperature outside of the standard temperature range, the optical amplification system can maintain output optical power and noise figure at the expense of power consumption, or the optical amplification system can minimize power consumption at the expense of output optical power and noise figure. If an uncooled pump laser is used to minimize power consumption, the optical amplification system cannot maintain the output optical power and noise figure over a wide temperature range. If a thermoelectric cooler is used to fix the pump temperature, the optical amplification system can maintain output optical power and noise figure, but the power consumption will increase.
FIG. 1 illustrates a graph of wavelength versus power for performance of a pump laser at different temperatures in the prior art. The wavelength output by a pump laser varies with the temperature of operation of the pump laser. This temperature-dependent variation limits the temperature range for operation of optical amplification systems using pump lasers. In order to hold the wavelength of a pump laser stable, the optical amplification system uses a fiber Bragg grating. The fiber Bragg grating will effectively lock the pump laser wavelength as long as the wavelength peak of the gain medium in the laser is close to the center wavelength of the fiber Bragg grating. The wavelength peak of the gain medium is fundamentally temperature dependent. Therefore, if the temperature of the gain medium is not controlled by using a temperature control device, such as a thermoelectric cooler, the gain medium wavelength will drift sufficiently far away from the fiber Bragg wavelength so as to cause the pump laser to “unlock” and lase at a frequency other than the desired one. Thus, one resulting problem is a dramatically reduced gain from the optical amplification system.